Cement kiln dust is a by-product of the cement manufacturing process that is removed from the stream of kiln gases as they pass through the kiln's dust collection system during clinker production. Cement kiln dust can vary in composition from virtually unaltered kiln feed to over 90% alkali sulfates and chlorides depending on process type, kiln configuration, raw materials, fuels, process characteristics, and points of collection. It can vary in particle size from that of fine sand or silt to that of clay, with particle size distribution ranging from very broad to very narrow depending on material and process parameters. The quantities of dust generated from a particular kiln depends on the factors that control cement kiln dust composition as well as the internal configuration of the kiln, the quantities of gasses passed through the kiln, and other operating conditions.
Cement kiln dust is a major problem at many cement manufacturing plants. Dust is generated in large quantities and is often not suitable for direct return to the cement-producing process as a feed because of high concentrations of alkali metals and sulfates, and incompatibility of the dust with the process. Since large quantities of dust cannot be returned directly to the kiln, it must be disposed of in a safe manner. General disposal practices are placement of dust in waste piles or in land-or quarry fills. Such disposal methods are inherently unsatisfactorily because they involve wasting a material for which significant processing and handling costs and efforts have been incurred. Since environmental regulations have matured, the costs and problems of disposal have become more onerous and continued disposal of kiln dust has become more expensive.
When cement kiln dust is brought into contact with water, high concentrations of anhydrous phases, which include oxides, sulfates and chlorides, are soluble and leached. Since the prime source of cement kiln dust instability is the high contribution of alkali metals oxides, and sulfates, which have high affinity towards water, the question is how to convert the undesired oxides into stable materials like carbonates, to decrease the solubility, and consequently increase the durability of the application, through effective and inexpensive processes.
Treated cement kiln dust has the potential for use in engineering projects such as soil stabilization, waste stabilization/solidification, Portland cement replacement, asphalt pavement, controlled low strength material (flowable fill), Pozzolanic activator, lightweight aggregate, and construction fill, but this isn't always possible.
Problems relating to cement kiln dust have long been recognized, and various methods have been proposed for their solution. The following methods have been suggested for treating cement kiln dust. The methods include leaching the dust with water to remove alkalis:
Nestell, in a U.S. Pat. No. 1,307,920 mixed kiln dust with water and passed carbon dioxide into the resulting mixture to substantially neutralize the slurry. However the product could not be recycled back into the cement kiln for its use as a kiln feed material unless the alkali levels of the original dust were very low.
Palonen et al., in U.S. Pat. No. 2,871,133 agglomerated cement kiln dust at high pressure and temperature, to render the alkalis more soluble. The resulting heat-treated agglomerate was then leached with water to remove the soluble alkalis. The residual solids are further treated to adjust moisture for return to a cement kiln. This process suffers because it is very complicated.
Patzias, in U.S. Pat. No. 2,991,154 mixed kiln dust with water and then heated at a known pressure. The slurry was filtered to separate the solution containing the alkalis from the residual solids. Then the separated solution was treated by neutralization with sulfuric acid, evaporation, centrifugation, or a combination thereof, to recover alkali sulfates, for recycling to the cement-making process. This process is not practical because of high water to dust ratio, high temperature, and high pressure to affect the dissolution of alkalis. Kiln dust solids would differ significantly in composition from normal kiln feed requiring kiln feed correction.
McCord, in U.S. Pat. No. 4,031,184 leached cement kiln dust at high temperature (but not at high pressure) using potassium chloride to enhance solubility. Then, the cement kiln dust solids are flocculated using oil and a fatty acid and the precipitates are palletized. Since the solubility of potassium chloride is higher than that of potassium sulfate by more than a factor of two in both hot and cold water, it is much more likely that any precipitate will be potassium sulfate rather than potassium chloride.
Helser, et al, in U.S. Pat. No. 4,219,515 added carbon dioxide to wastewater from the production of hydrous calcium silicates from lime and silica in order to remove calcium from the water so that it can be recycled to the production process. The resulting calcium carbonate precipitate presumably can be re-producing lime.
Kachinski, in U.S. Pat. No. 4,402,891 added water to cement kiln dust in a carbon dioxide atmosphere. Alkalies are not completely removed, and the material is not suitable for return to a cement-making process.
Neilsen, in U.S. Pat. No. 5,173,044 used wet-process slurry to scrub sulfur from kiln gases and retain them in the kiln. This process is of limited applicability because it retains all of the alkalies in the kiln so that, in the majority of cases, only limited amounts of cement kiln dust can be used.
Brentrup, in U.S. Pat. No. 5,264,013 collected cement kiln dust in a conventional dust collector, which was later progressively heat-treated to volatilize low-boiling pollutants and collect them with a carbonaceous filter medium. The ability to return cement kiln dust to the cement-making process was not enhanced.
Huege, in U.S. Pat. No. 5,792,440 used carbon dioxide to treat a supernatant liquid after leaching and separation of the solids from lime kiln dust in order to produce high purity precipitated calcium carbonate as a separate product, for treating flue gases exhausted from a lime kiln. This method is only useful as an effluent control.
Gebhardt, in U.S. Pat. No. 6,331,207 moistened the supply of cement kiln dust with carbon dioxide to convert the materials to carbonates. During the carbonation cycle, the water in the hydroxides is released to formulate slurry. The soluble alkalis and sulfate are released in the liquid phase with the solids being separated from the liquid. Then, the solids were washed to provide a useful feed to the kiln while, the liquid contains alkali salts.
Prior methods that have been used in the past frequently suffer from the following problems:
1. Only part of the alkalis are readily soluble, often half or less.
2. Typical ratios of water to dust are 10:1 to 20:1, or higher.
3. An effluent, high in pH (>10) and dissolved solids, are discharged.
4. Dissolved solids tend to precipitate in the receiving waters.
5. The high pH effluent is detrimental to the biosphere.
6. The recovered solids are high in water content, often over 70%.
7. Adjustments to kiln feed chemistry may be required when treated dust is returned to the kiln.
These problems are so severe that the leaching methods of the past have been largely banned by Environmental Protection bodies.
The present invention provides a new technique for producing treated cement kiln dust through reduction of alkali metals and sulfates solubility's by conditioning the dust so that it is compatible with the process to which it will be introduced. The highly alkaline waste kiln dust found in the forms of oxides and hydroxide of calcium, potassium and sodium is converted to mildly alkaline calcium carbonate, and potassium and sodium bicarbonates.